Human blood is classified by antigenic compatibility according to two main systems: the ABO system and Rhesus (Rh) system. The Rh system consists of several closely related proteins encoded by two tandemly duplicated genes on human chromosome 1. Clinically, the most important Rh antigen is called Rhesus D (RhD) antigen. Humans who have this antigen on their red blood cells are termed RhD positive (RhD+), whereas those that do not carry the antigen are termed RhD negative (RhD−). As the Rh blood types are inherited characteristics, a fetus may inherit a blood type from its father that differs from that of its mother. Clinically this is not a problem if the mother is RhD+, but if the mother is RhD− and the fetus inherits the dominant RhD allele from the father, complications during pregnancy may arise.